Windows: AFSDeleteObjectInfo InterlockedExchange

Use InterlockedExchangePointer to disconnect the ObjectInformationCB
pointer from the caller.  This ensures that only one thread can
successfully call AFSDeleteObjectInfo on the same object at a time.

Change-Id: Ie70f52fc443f88c3cb4be41f12caa91466d92905
Reviewed-on: http://gerrit.openafs.org/9644
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>